Bending device for thin-walled metal pipes

ABSTRACT

A bending device is described for thin-walled metal tubes (R) made of a straight tube guide ( 1 ), a core template ( 2 ), which adjoins the tube guide ( 1 ) and is pivotable in relation thereto, having a connection strip ( 3 ), and a mandrel, having a mandrel tip ( 4 ), which is flexible in the region of the core template ( 2 ), and a mandrel shaft ( 5 ), which adjoins the mandrel tip ( 4 ) and is rigidly connected thereto and is axially fixed ( 1 ) in the tube guide. In order to support the metal tube (R) to be bent over its entire surface on the inside in the active bending region during the bending procedure, the flexible mandrel tip ( 4 ), which comprises a radially incompressible solid cylinder made of elastomer, extends only over the starting section of the bending curve, where the bending procedure occurs. In order to pull the metal tube (R) to be bent over this mandrel tip ( 4 ), the tube is clamped in the region of the core template ( 2 ) for a short length using a spherical clamping head ( 7 ) insertable in the metal tube (R), the clamping head ( 7 ) being movable in relation to the mandrel tip ( 4 ) in the pivot region of the core template ( 2 ) and being attached to a traction means ( 6 ) implemented as a retrieval element.

BACKGROUND OF THE INVENTION

The present invention relates to a bending device for thin-walled metaltubes made of a straight tube guide, a core template, which adjoins thetube guide and is pivotable in relation thereto, having a connectionstrip, and a mandrel, having a mandrel tip, which is flexible in theregion of the core template, and a mandrel shaft, which adjoins themandrel tip and is rigidly connected thereto and is axially fixed in thetube guide.

Bending devices of this and similar types for thin-walled tubes areknown (the magazine “Werkstatt und Betrieb” 104 (1971) 4, pp. 271 to274, “Verhinderung von Faltenbildung und Einknicken dünnwandiger Rohrebeim Biegen [Prevention of Wrinkling and Buckling of Thin-Walled TubesDuring Bending]” by Prof. Dr.-Ing. G. Öhler, Bad Dürkheim).“Thin-walled” is understood in this context to mean tubes whose wallthickness is small in relation to the diameter and the bending radius,for example, tubes having a wall thickness of approximately 0.8 mm at adiameter of approximately 80 mm and a bending radius of approximately120 mm. In order to keep the problems of wrinkling and deviation from acircular profile, which arise during bending of such tubes, as small aspossible, the tube has been supported on its inner wall over the entirelength of the bending curve using mandrels fixed axially in the tube.Therefore, there are mandrels supporting the tube over the entirebending curve which have a bendable section in the form of helicalsprings having turns pressed tightly together or in the form of balllinks which form a link chain (U.S. Pat. No. 4,481,803). Both types ofmandrels have the disadvantage that the tube to be bent is not supportedover its complete surface in the region where the tube is bent. Mostcritical is the incomplete support on the inner curve, where wrinklesmay form. With linked mandrels made of ball links, gaps are present fromthe beginning on the inner curve, so that the danger of wrinkling isparticularly great for them.

A further problem in the known bending devices is the clamping of thebeginning of the tube. Support on a rigid cylindrical mandrel head isproblematic, because such a mandrel head, which requires a relativelylarge clamping length for fixed clamping of the tube, cannot be pulledback through the bent tube.

SUMMARY OF THE INVENTION

The present invention is based on the object of providing a bendingdevice of the type initially described which manages with a shortclamping length of the beginning of the tube and which allows theproduction of a curve having a circular cross-section over its entirelength.

This object is achieved using a bending device of the type initiallydescribed in that the mandrel tip is implemented as a solid cylinder,made of an elastomer, whose external contour is tailored to the innercontour of the metal tube to be bent and which is practicallyincompressible radially, and which extends over only the startingcross-section of the bending curve, and a spherical clamping head,insertable into the metal tube, is provided in the region of the coretemplate and its connection strip, this clamping head being movable inrelation to the mandrel tip in the pivot range of the core template andbeing attached to a traction means implemented as a retrieval element.

In the bending device according to the present invention, the metal tubeis supported on the inside “all over” in the starting section of thebending curve, which is active in bending, by the flexible, but radiallypractically incompressible solid cylinder made of elastomer, so that inthis section, which is decisive for the shaping, no constrictions orwrinkles may form. This is essentially true even if the solid cylinder,for the purpose of improving its flexibility, has radial slots at leaston the outside in the bending curve or is constructed from disks. Thisis because, in contrast to mandrel tips constructed from helical screwsor link chains, the radial slots in a mandrel tip constructed from asolid cylinder made of elastomer open comparatively slightly due to theelastomer. Optimum internal support using disks may be achieved with apurposely adjustable flexibility if elastomer disks having differingmodules of elasticity are used. In the region further toward the mandreltip, a certain radial compressibility may be very advantageous for thebending result, because better flexibility is, as a rule, connectedtherewith. Using the clamping head, at least rounded on both ends, andparticularly spherical, which is moved with the core template as thecore template pivots, the metal tube may be sufficiently stronglyclamped even on a shorter length. There are also no problems whenpulling back the mandrel after the bending procedure is complete,because the clamping head may pass the bending curve without problems,particularly without jams, due to its short length and spherical shape.In addition, it may also have a smoothing and reprofiling effect on themetal tube.

In order to improve the flexibility of the solid cylinder in the bendingcurve, in addition to the possibility of radial slots or theconstruction from disks, particularly disks made of differentelastomers, there is also the possibility of providing axial slots, likea laminated mandrel. The flexural strength of the solid cylinder overits length may also be set easily in this way if a very few axial holesof different diameters and different depths are introduced into thesolid cylinder, distributed from the face out. Of course, a slightreduction of radial stiffness must be accepted in this case. It is thenleft to the decision of the practitioner whether any compromise is to bemade, and if so, how large a compromise. However, in all cases theslotted cross-section is preferably restricted to the region above theneutral fiber of the curved mandrel tip.

The present invention provides two alternative possibilities for theretrieval (pulling back) of the mandrel and the clamping head after thebending procedure is completed: according to the first alternative, thetraction means is at least flexible within the bending curve, runswithout play through the mandrel tip and is fixed in the mandrel tipand/or the mandrel shaft, and is also movably guided by the clampinghead up to a stop. In this case, a pressure spring is preferablyinserted between the clamping head and the stop, which, in the unloadedstate, keeps the clamping head pressed against the face of the mandrel.This starting position makes inserting the mandrel for preparing thebending procedure easier. According to the other alternative, on oneend, the traction means is flexible at least within the bending curveand runs through the mandrel tip and the mandrel shaft and, on the otherend, is connected to the clamping head. In this case, the retrievalforce may engage directly on the traction means.

There are also multiple possibilities for the implementation of theclamping head. The clamping head may comprise a slotted, sphericalexternal ring and an internal support and clamping cone, which has aself-clamping effect on the external ring during the bending procedure.In this alternative, the tractive force exercised on the tube during thebending procedure is transmitted to the clamping ring and the clampingcone and is used to expand the external ring and therefore to produce amore solid clamping effect, while the force exercised by the tractionmeans during retrieval of the clamping head acts to loosen it.Alternatively, the clamping head may also comprise multiple ball links,particularly two, which form a rigid unit. Due to the centralconstriction and the roundings, there are also no jams in the tube curveas the clamping head is pulled back in this case. This achievement ofthe object is distinguished above all by optimal guiding and especiallygood reprofiling properties, with a simple construction.

Since the traction means becomes more and more free between the mandreland the clamping head as the bend increases during the bendingprocedure, it is advantageous to have it run eccentrically through themetal tube toward the tube inner curve. The advantage is that thetraction means may support itself on the inner curve of the tube innerwall as it is pulled back.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the bending device at the end of the bending procedure inaxial cross-section.

FIG. 2 shows thebending device at the end of the bending procedure inaxial cross-section with the clamping head and the traction meansdifferent from that of the bending device shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the present invention is described in more detail withreference to a drawing illustrating two exemplary embodiments. Bothfigures show the bending device at the end of the bending procedure inaxial cross-section.

The bending device shown in FIG. 1 comprises a fixed straight tube guide1, an adjoining pivotable core template 2, having a connection strip 3,and a mandrel, having a mandrel tip 4 positioned in the region of coretemplate 2 and an adjoining mandrel shaft 5, rigidly attached to mandreltip 4 and located in the region of tube guide 1. Mandrel tip 4 andmandrel shaft 5 are axially fixed during the bending procedure. Mandreltip 4 comprises a solid cylinder made of elastomer, which is flexible,but, in regard to the deformability of the tube material, is practicallyincompressible radially. In practice, hardnesses in the magnitude from95 Shore A to 50 Shore D are suitable for steel tubes. To improve theflexibility, the solid cylinder may be slotted toward the outer curve inthe section above its neutral fiber F, which is active in bending, andmay have axial slots S1 (FIG. 1) or radial slots S2 (FIG. 2).

As traction means 6, a cable runs eccentrically offset through mandreltip 4 and mandrel shaft 5 toward core template 2 and therefore towardthe inner curve. Traction means 6 is axially fixed in mandrel shaft 5and is seated with slight play in mandrel tip 4, which is favorable forthe radial incompressibility of mandrel tip 4, unless a certaincompressibility is intentionally accepted to favor elevated flexibility,above all in the region toward mandrel tip 4, possibly even usingfurther axial holes of different cross-sections and different depths andat a sufficient distance from the external circumference, which may becompletely practical for elastomer which is initially veryincompressible. A pressure disk 8, which is axially fixed in a groove ontraction means 6 and minimizes axial deviation of the elastomer materialof the solid cylinder, consequently reducing its diameter, pressesagainst the face of mandrel tip 4. Traction means 6 extends furtherthrough a clamping head 7. A pressure spring 9 is seated on tractionmeans 6 between clamping head 7 and a stop 6 a. Clamping head 7comprises a slotted, spherical external ring 7 a and an internal supportand clamping cone 7 b. Tube R to be bent may be clamped over a shortlength between external ring 7 a and the straight section of coretemplate 2 and connection strip 3.

The mode of operation of the bending device according to the presentinvention in the exemplary embodiment of FIG. 1 is as follows:

Tube R to be bent is inserted, with the straight section of coretemplate 2 in an aligned position with tube guide 1. Subsequently, themandrel, having mandrel tip 4 and mandrel shaft 5, with clamping head 7pressed against mandrel tip 4, is inserted into tube R until mandrel tip4 lies in the region of core template 2. Connection strip 3 is thentensioned using core template 2 and clamping head 7, through which tubeR is clamped. Mandrel shaft 5 is axially fixed in tube guide 1 in thisaxial position by means not shown in the drawing.

If core template 2 is now pivoted counterclockwise, then tube R, clampedby core template 2 and connection strip 3, is carried along and drawnover axially fixed mandrel 4. Due to the conical seat of clamping head7, the tractive forces, which have a self-clamping effect on externalring 7 a, ensure that the clamping force is amplified. Tube R reachesthe curved region of core template 2 and initially straight mandrel tip4 becomes more and more curved, tube R being continuously supported onits inner wall in the active bending region during the bendingprocedure, so that no wrinkles or constrictions may form.

After the bending procedure is completed, connection strip 3 is loosenedand the mandrel, with clamping head 7, is pulled back through bent tubeR. At the same time, the clamping effect of clamping head 7 due to theconical seat is removed because of the forces now acting in the oppositedirection. At the end of this procedure, pressure spring 9 pressesclamping head 7 back into its starting position on the face of mandreltip 4. Bent tube R may then be removed and core template 2 may bepivoted back into its starting position, so that a new tube to be bentmay be inserted.

The exemplary embodiment of FIG. 2 differs from that of FIG. 1 only inthe clamping head and the traction means. In cases of correspondence,the same reference numbers are used for the individual parts as in FIG.1.

Clamping head 17 comprises multiple, particularly two, ball links,rigidly connected to one another, which are connected to a cable astraction means 16. Alternatively, clamping head 17 may also comprise aone-piece, rigid, short body having an external contour similar to abellows. Traction means 16 is guided movably through mandrel tip 4 andmandrel shaft 5. During pivoting, it may therefore be loosened. For theretrieval, the retrieval force engages on traction means 16, which isguided outward. In this case, clamping head 17 is first pulled backthrough the tube until it stops against the face of the mandrel tip.Clamping head 17 is then pulled back together with mandrel tip 4 andmandrel shaft 5. Jamming in the tube curve may not occur, in spite ofthe rigid unit of the ball links, due to their good axial guiding onaxially offset positions and their shape. Their roundings and theircentral constriction provide sufficient clearance for the curved tubeand also have a reprofiling effect on tube R as they are retrieved. Ofcourse, if there are more than two ball links, e.g., three links, thecontour of the central link must be set back at least at the innercurve. This is correspondingly true for a clamping head similar to abellows.

1. A bending device for bending thin-walled metal tubes, comprising: astraight tube guide, a core template which adjoins the tube guide and ispivotable in relation thereto, said core template having a connectionstrip, a mandrel having a mandrel tip which is flexible in a region inproximity to said core template, and a mandrel shaft which adjoins themandrel tip and is rigidly connected thereto and is axially fixed insaid tube guide, wherein said mandrel tip comprises a solid cylindermade from an elastomer, said mandrel tip having an external contourwhich conforms to an inner contour of a metal tube to be bent, saidmandrel tip being substantially incompressible radially, said mandreltip extending into only a starting cross-section of a bending curvedefined by the mandrel, said mandrel further comprising a clamping headwhich is insertable into the metal tube to be bent, said clamping headbeing positioned in a region of the core template and the connectionstrip, said clamping head being movable relative to the mandrel tip in apivot range of the core template, and a traction unit attached to saidclamping head, said traction unit comprising a retrieval element forsaid clamping head.
 2. The bending device of claim 1, wherein themandrel tip includes radial or axial slots.
 3. The bending device ofclaim 2, wherein the mandrel tip comprises elastomer disks held againstone another by axial pre-tension in a face region of said mandrel tip.4. The bending device of claim 3, wherein said elastomer disks have avariety of modules of elasticity.
 5. The bending device of claim 1,further comprising a pressure disk located on a face of the mandrel tip.6. The bending device of claim 1, wherein the retraction unit isflexible at one end in the bending curve up to the mandrel tip, and isfixed in the mandrel tip substantially without play or is fixed to themandrel shaft, and on an opposite end is movably guided through theclamping head up to a stop.
 7. The bending device of claim 6 furthercomprising a pressure spring located between the clamping head and thestop.
 8. The bending device of claim 1, wherein on one end, the tractionunit is flexible at least in the bending curve and runs movably throughthe mandrel tip and the mandrel shaft, and on an opposite end isconnected to the clamping head.
 9. The bending device of claim 1,wherein the clamping head comprises a slotted external ring and aninternal support and clamping cone which, during bending, has aself-clamping effect on the external ring.
 10. The bending device ofclaim 1, wherein the clamping head comprises multiple ball links whichform a rigid unit.
 11. The bending device of claim 1, wherein thetraction means runs eccentrically through the metal tube to be benttowards an inner curve of the metal tube.